1. Field of the Invention
The invention relates to nematic liquid crystal devices formed with anisotropic protrusions, and methods of production. thereof.
2. Description of the Related Art
The invention can be used to provide an improvement in obtaining and if necessary isolating the operating state of a pi-cell liquid crystal device at low voltages. The invention can also be applied to nucleating the operating states from the globally stable states in the BTN device, and for isolating the operating states from each other and from the globally stable state.
The invention is particularly applicable to flat panel displays and pi-cells used in LCTV. Another application is in using BTN displays in mobile products.
P. D. Berezin, L. M. Blinov, I. N. Kompanets and V. V. Nikitin ‘Electro-optic Switching in Oriented Liquid-Crystal Films’ July-August 1973 Sov. J. Quant. Electron Vol. 3, pp. 78-79 is a paper which describes the achievement of fast nematic liquid crystal response times obtained in low surface tilt, non-twisted cells. It is unclear from the paper whether the alignment directions at the two surfaces are parallel or anti-parallel. The fast switching times were achieved by applying a bias voltage such that the optical modulation is mostly caused by reorientation of the liquid crystal molecules near the surface region, whilst those in the bulk remained substantially homeotropic. (See for example lines 11-19, column 1, page 79, together with lines 14-17, column 2, page 78)
P. J. Bos and K. R. Koehler/Beran ‘The pi-Cell: A Fast Liquid Crystal Optical Switching Device’ 1984 Mol. Cryst. Liq. Cryst. Vol. 113, pp. 329-339 is a paper which introduces the pi-cell as a liquid crystal display mode that is fundamentally fast, since the symmetry of the device eliminates the adverse effects of a flow induced “backwards” torque exerted on the liquid crystal director towards the cell centre. The pi-cell has parallel surface tilt (the term “parallel” here referring to the rubbing directions at the two cell surfaces, rather than to the directors) such that at low voltages the splay (or H-) state is stable (see FIG. 3b, page 332). At higher voltages switching takes place between a high and low voltage bend (or V-) state (see FIG. 2b, page 331).
U.S. Pat. No. 4,566,756 describes a pi-cell doped for d/p>0.25. As a result of the doping this device does not form the H-state at low voltages and remains in the T-state, not the V-state at high voltages. However at high voltages the T-state has similar optical properties to that of the V-state. This mode then overcomes the nucleation problems of the conventional pi-cell and demonstrates similar optical characteristics at high voltages. Unfortunately at lower voltages the effect of the inherent twist on the optical characteristic gives a poorer performance than the conventional pi-cell.
EP 0,996,028 describes the generic use of nucleation regions to initiate the growth of the V-state from the H-state. Particular examples are given of the pi-cell device and the complementary negative pi-cell or SBD. As well as teaching patterned surface pre-tilt to generate these nucleation regions, it also teaches in-situ polymerisation of a nucleation region e.g. in the inter-pixel gaps. One technique described for patterning surface tilt is to use patterned reactive mesogen layers. These are however thin and do not protrude considerably into the liquid crystal.
EP 0,965,876A2 describes the use of substantially homeotropic alignment to surround the active pixel area within a conventionally low surface pretilt pi-cell based display. At zero volts the surrounding substantially homeotropic alignment isolates the pixel from the reformation of the H-state, at zero volts then the pixel relaxes into the twist state (T-state) (see abstract column 2, page 3, lines 39-44 and claim 1). The device is operated at higher voltages in the V-state.
Japanese Patent Application JP-A-9 90432 (Toshiba) discloses the provision of nucleation sites within a pi-cell panel. The nucleation sites are provided by including spacer balls or pillars within the pi-cell panel and cooling the liquid crystal material from an isotropic phase to a nematic phase while an electric field is applied across the panel. This results in some of the spacer balls or pillars acting as nucleation sites for growth of the V-state. However the positioning of the spacers cannot be easily controlled.
Japanese Patent Application JP-A-9 218411 (Sekisui) discloses a LCD having a bend alignment state, which is stabilised, in the absence of an applied field, by the presence of spacers in the form of spherical particles The spacers have a surface energy such that liquid crystal molecules adjacent the alignment layers are mainly aligned parallel to the alignment layers. However, in order for this technique to work, a field has to be applied during the initial alignment of the device. Also, the particles can not be positioned so as to be outside the pixel apertures so that the contrast ratio of the display is reduced by the presence of the particles.
European Patent Application No. 00310110.2 describes the use of birefringent spacers (for example those made from polymerisable reactive mesogens) in a liquid crystal display, said spacers having the same optical characteristic as one of the states of the switched states of the liquid crystal display.
DE 2,849,402A1 describes a display with both variable and invariable parts. The variable parts are liquid crystal, whilst the invariable parts are fabricated from a birefringent material running between the top and bottom surfaces. These invariable parts act also as spacers.
British Patent Application No. 9704623.9 describes a liquid crystal device in which at least one alignment layer has been fabricated from a mixture of mono-functional and di-functional reactive mesogens, the ratio of these materials acting to control the surface pre-tilt of the contiguous liquid crystal material. It teaches that as the percentage of mono-functional reactive mesogen is increased then the tilt at the RM/air interface is increased.
U.S. Pat. No. 5,995,184 describes the fabrication of thin film retardation plates from polymerisable reactive mesogens. It teaches the addition of an additive that migrates to the RM/air interface to modify the surface energy and hence control the tilt of the reactive mesogen molecules at that surface, in particular it teaches reducing the tilt angle at the RM/air interface.
EP 0,880,052 describes the fabrication of multi-domain liquid crystal devices by using a patterned, thin, twisted reactive mesogen layer above a conventional alignment layer. Although FIG. 5 of the British Application (9710481.4) shows parallel alignment in the none reactive mesogen region and twisted alignment above the reactive mesogen region, the regions above the reactive mesogen are not used for nucleating an operating state from a zero voltage non-operating state. This is additionally supported by the regions above the RM being of the same area as regions not above the RM, and that the application explicitly teaches to make the RM layer as thin as possible.
EP 0,018,180 and Journal of Applied Physics Vol. 52, No. 4, p. 3032 (1981) disclose the basic operating principle of the bistable twisted nematic (BTN) device i.e. switching between two metastable states of twist angle ±180°, a state of angle ° being the globally stable state not being used. These also disclose the use of isolation by the reduction of cell thickness in the unaddressed regions. A BTN device is a slower device than a pi-cell, but is stable in the two states of 100 ±180°, hence the name bistable. A typical BTN device is referred to as having antiparallel alignment because the rubbing directions at the two liquid crystal surfaces are anti-parallel. This actually results in the directors at the two surfaces of the liquid crystal being aligned parallel to each other.
British Patent Application No. 9911730.1 describes the fabrication of BTN devices with an isolation region between neighbouring pixels, the isolation region is a HAN state, a uniform lying helix or a focal conic texture. It is formed through patterning the surface pretilt of the alignment layer, such that the pretilt of interpixel gaps is different from that of the intrapixel region.
The antiparallel BTN has three possible stable twist states e.g. 0°, 180° and 360°, more generally: (phi−pi), phi and (phi+pi) twist states. With suitable addressing, two of these states can be accessed and used as operating states. The energy of the three stable twist states depends on the thickness-to-pitch (d/p) ration and differs, such that the lowest energy state, which is normally topologically distinct to the other two stable twist states, does not correspond to an operating state, e.g. phi. This results in the energetically favourable (but undesirable) state nucleating and growing over a period of time once the voltage is turned off. Even if this undesired state does not nucleate when the voltage is turned off, bistability is not possible without isolation because the more energetically favourable of the operating states will slowly grow into the other one.